Is (NH 3) 2 hydrogen bonded?

Abstract

Vibration-rotation-tunneling (VRT) splittings have been computed for the dimer (NH 3) 2 by the use of four different model potentials. The six-dimensional nuclear motion problem is solved variationally in a symmetry adapted basis consisting of analytic radial functions and rigid rotor functions depending on the five internal angles, as well as on the three overall rotation angles. Two of the potentials are designed such that they have no barrier for interchange tunneling and the other two potentials have barriers of 31.1 and 24.4 cm −1, respectively. The top of the barrier corresponds to a cyclic structure and the two equivalent minima on either side of the barrier to nearly linear hydrogen bonds. Energy splittings, dipole moments, nuclear quadrupole splittings, and the amount of quenching of the monomer umbrella inversions are computed and compared with the available experimental numbers. The potential that gives best agreement with the observed quantities has an equilibrium hydrogen bonded structure close to linear, but a VRT-averaged ground state structure that is nearly cyclic.